Solid-State Dynamics in the closo-Carboranes:A (11)B MAS NMR and Molecular Dynamics Study

This work explores the dynamic behaviour of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the 11B MAS linewidths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tauc) calculated by molecular dynamics are on the picosecond timescale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the 11B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the 11B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQeff derived from “dynamic shift” measurements using 11B MQMAS NMR spectroscopy. The behaviour of PQeff as a function of temperature for para-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures and this was supported by the molecular dynamics simulations

The determination of all six spectral densities of motion for partially oriented dichloromethane-d2

Measurements of spin-lattice relaxation, combined with 2-dimensional measurements of single-and double-quantum spin-echo decay rates, were used to detn. the complete set of spectral d. parameters which characterize D relaxation of CD2Cl2 partially oriented in a nematic liq. crystal (Merck Licristal Phase V). At 31.5 Deg and 9.2 MHz, terms describing cross-correlation between 2 D quadrupole coupling tensors are comparable to the autocorrelation terms. A partial interpretation of these results in terms of anisotropic mol. reorientation and order-director fluctuations is presented and is confirmed by addnl. measurements of the temp. and field dependence of spin-lattice relaxation rates. [on SciFinder (R)

Synthesis and dynamic nmr of hexathiadodecamethoxymetacyclophane

A “one-pot” synthesis of the title compound (1) is described, which might also be suitable for other related compounds. It is shown by 1H NMR that (1) has a saddle structure with C2 symmetry. At room temperature it undergoes fast dynamic interconversion between two symmetry related structures resulting in an overall four-fold symmetry. The activation parameters for this reaction are: ΔE#=10.5 kcal/mole, ΔS#=7.1 e.u. At ~-100°C the reaction is frozen out on the NMR time scale

The single-rotation method of determining quadrupole coupling tensors in monoclinic crystals. Error analysis and application to bullvalene

Traceless symmetric second-rank tensors such as quadrupole coupling (QC) tensors can be determined from data (line splittings in the case of QC tensors of spin-1 nuclei) collected from rotating crystals about two axes provided these are neither parallel nor orthogonal. We give a simple argument for the latter condition. Rotating a monoclinic crystal about a single axis provides, in general, data for two axes and is sufficient for determining all five independent components of QC tensors. The accuracy of the results of this so-called single-rotation method, however, depends sensitively on the choice of the rotation axis. Here we give an error analysis of this method, focusing attention on the errors introduced by the uncertainty in the direction of the rotations axis. We apply the single-rotation method for measuring the 10 independent deuteron QC tensors in bullvalene, of which only one 50% deuteron-enriched single crystal was available. The challenge lies in the need to disentangle the dependence of 40 resonances on the rotation angle while in no spectrum could more than 36 peaks be resolved. The measured QC tensors are discussed in terms of the local symmetry of the deuteron sites and the approximate C3v, symmetry of the bullvalene molecule in its crystal environment. They provide a reliable basis for studying the complex dynamics of bullvalene in the solid state by deuteron NMR lineshape analysis and 2D exchange spectroscopy